Cross-Component Transform Coefficient Level Reconstruction

ABSTRACT

This disclosure relates to cross component methods for refining decoded transform coefficients before or after dequantization in video decoding. For example, a method for video decoding is disclosed. The method may include, comprising extracting a first transform coefficient of a first color component from a bitstream of a coded video; extracting a second transform coefficient of a second color component from the bitstream of the coded video; deriving an offset value based on a magnitude or sign value of the first transform coefficient; adding the offset value to a magnitude of the second transform coefficient to generate a modified second transform coefficient for the second color component; and reconstructing the coded video based on at least the first transform coefficient of the first color component and the modified second transform coefficient of the second color component.

INCORPORATION BY REFERENCE

This application is based on and claims the benefit of priority to U.S.Provisional Application No. 63/224,046, entitled “CROSS-COMPONENTTRANSFORM COEFFICIENT LEVEL RECONSTRUCTION”, filed on Jul. 21, 2021,which is herein incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to a set of advanced videocoding/decoding technologies and more specifically to cross componentmethods for refining decoded transform coefficients before or afterdequantization.

BACKGROUND

This background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing of thisapplication, are neither expressly nor impliedly admitted as prior artagainst the present disclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, with each picture having a spatialdimension of, for example, 1920×1080 luminance samples and associatedfull or subsampled chrominance samples. The series of pictures can havea fixed or variable picture rate (alternatively referred to as framerate) of, for example, 60 pictures per second or 60 frames per second.Uncompressed video has specific bitrate requirements for streaming ordata processing. For example, video with a pixel resolution of1920×1080, a frame rate of 60 frames/second, and a chroma subsampling of4:2:0 at 8 bit per pixel per color channel requires close to 1.5 Gbit/sbandwidth. An hour of such video requires more than 600 GBytes ofstorage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the uncompressed input video signal, through compression.Compression can help reduce the aforementioned bandwidth and/or storagespace requirements, in some cases, by two orders of magnitude or more.Both lossless compression and lossy compression, as well as acombination thereof can be employed. Lossless compression refers totechniques where an exact copy of the original signal can bereconstructed from the compressed original signal via a decodingprocess. Lossy compression refers to coding/decoding process whereoriginal video information is not fully retained during coding and notfully recoverable during decoding. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is made smallenough to render the reconstructed signal useful for the intendedapplication albeit some information loss. In the case of video, lossycompression is widely employed in many applications. The amount oftolerable distortion depends on the application. For example, users ofcertain consumer video streaming applications may tolerate higherdistortion than users of cinematic or television broadcastingapplications. The compression ratio achievable by a particular codingalgorithm can be selected or adjusted to reflect various distortiontolerance: higher tolerable distortion generally allows for codingalgorithms that yield higher losses and higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories and steps, including, for example, motion compensation,Fourier transform, quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, a picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be referred to as an intra picture. Intra pictures and theirderivatives such as independent decoder refresh pictures, can be used toreset the decoder state and can, therefore, be used as the first picturein a coded video bitstream and a video session, or as a still image. Thesamples of a block after intra prediction can then be subject to atransform into frequency domain, and the transform coefficients sogenerated can be quantized before entropy coding. Intra predictionrepresents a technique that minimizes sample values in the pre-transformdomain. In some cases, the smaller the DC value after a transform is,and the smaller the AC coefficients are, the fewer the bits that arerequired at a given quantization step size to represent the block afterentropy coding.

Traditional intra coding such as that known from, for example, MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt coding/decoding of blocks based on, for example, surroundingsample data and/or metadata that are obtained during the encoding and/ordecoding of spatially neighboring, and that precede in decoding orderthe blocks of data being intra coded or decoded. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction uses reference data only from the currentpicture under reconstruction and not from other reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques are available in a given video coding technology,the technique in use can be referred to as an intra prediction mode. Oneor more intra prediction modes may be provided in a particular codec. Incertain cases, modes can have submodes and/or may be associated withvarious parameters, and mode/submode information and intra codingparameters for blocks of video can be coded individually or collectivelyincluded in mode codewords. Which codeword to use for a given mode,submode, and/or parameter combination can have an impact in the codingefficiency gain through intra prediction, and so can the entropy codingtechnology used to translate the codewords into a bitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). Generally, for intra prediction, a predictor block can be formedusing neighboring sample values that have become available. For example,available values of particular set of neighboring samples along certaindirection and/or lines may be copied into the predictor block. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions specified in H.265's 33 possible intra predictordirections (corresponding to the 33 angular modes of the 35 intra modesspecified in H.265). The point where the arrows converge (101)represents the sample being predicted. The arrows represent thedirection from which neighboring samples are used to predict the sampleat 101. For example, arrow (102) indicates that sample (101) ispredicted from a neighboring sample or samples to the upper right, at a45 degree angle from the horizontal direction. Similarly, arrow (103)indicates that sample (101) is predicted from a neighboring sample orsamples to the lower left of sample (101), in a 22.5 degree angle fromthe horizontal direction.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “5”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are example referencesamples that follow a similar numbering scheme. A reference sample islabelled with an R, its Y position (e.g., row index) and X position(column index) relative to block (104). In both H.264 and H.265,prediction samples adjacently neighboring the block under reconstructionare used.

Intra picture prediction of block 104 may begin by copying referencesample values from the neighboring samples according to a signaledprediction direction. For example, assuming that the coded videobitstream includes signaling that, for this block 104, indicates aprediction direction of arrow (102)—that is, samples are predicted froma prediction sample or samples to the upper right, at a 45-degree anglefrom the horizontal direction. In such a case, samples S41, S32, S23,and S14 are predicted from the same reference sample R05. Sample S44 isthen predicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has continued to develop. In H.264 (year 2003), for example,nine different direction are available for intra prediction. Thatincreased to 33 in H.265 (year 2013), and JEM/VVC/BMS, at the time ofthis disclosure, can support up to 65 directions. Experimental studieshave been conducted to help identify the most suitable intra predictiondirections, and certain techniques in the entropy coding may be used toencode those most suitable directions in a small number of bits,accepting a certain bit penalty for directions. Further, the directionsthemselves can sometimes be predicted from neighboring directions usedin the intra prediction of the neighboring blocks that have beendecoded.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions in various encoding technologies developed overtime.

The manner for mapping of bits representing intra prediction directionsto the prediction directions in the coded video bitstream may vary fromvideo coding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions for intro prediction that arestatistically less likely to occur in video content than certain otherdirections. As the goal of video compression is the reduction ofredundancy, those less likely directions will, in a well-designed videocoding technology, may be represented by a larger number of bits thanmore likely directions.

Inter picture prediction, or inter prediction may be based on motioncompensation. In motion compensation, sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), may be used for a prediction of a newly reconstructedpicture or picture part (e.g., a block). In some cases, the referencepicture can be the same as the picture currently under reconstruction.MVs may have two dimensions X and Y, or three dimensions, with the thirddimension being an indication of the reference picture in use (akin to atime dimension).

In some video compression techniques, a current MV applicable to acertain area of sample data can be predicted from other MVs, for examplefrom those other MVs that are related to other areas of the sample datathat are spatially adjacent to the area under reconstruction and precedethe current MV in decoding order. Doing so can substantially reduce theoverall amount of data required for coding the MVs by relying onremoving redundancy in correlated MVs, thereby increasing compressionefficiency. MV prediction can work effectively, for example, becausewhen coding an input video signal derived from a camera (known asnatural video) there is a statistical likelihood that areas larger thanthe area to which a single MV is applicable move in a similar directionin the video sequence and, therefore, can in some cases be predictedusing a similar motion vector derived from MVs of neighboring area. Thatresults in the actual MV for a given area to be similar or identical tothe MV predicted from the surrounding MVs. Such an MV in turn may berepresented, after entropy coding, in a smaller number of bits than whatwould be used if the MV is coded directly rather than predicted from theneighboring MV(s). In some cases, MV prediction can be an example oflossless compression of a signal (namely: the MVs) derived from theoriginal signal (namely: the sample stream). In other cases, MVprediction itself can be lossy, for example because of rounding errorswhen calculating a predictor from several surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 specifies, described below is atechnique henceforth referred to as “spatial merge”.

Specifically, referring to FIG. 2 , a current block (201) comprisessamples that have been found by the encoder during the motion searchprocess to be predictable from a previous block of the same size thathas been spatially shifted. Instead of coding that MV directly, the MVcan be derived from metadata associated with one or more referencepictures, for example from the most recent (in decoding order) referencepicture, using the MV associated with either one of five surroundingsamples, denoted A0, A1, and B0, B1, B2 (202 through 206, respectively).In H.265, the MV prediction can use predictors from the same referencepicture that the neighboring block uses.

SUMMARY

Aspects of the disclosure provide cross-component methods andapparatuses for for refining decoded transform coefficients before orafter dequantization in video decoding. In some example implementations,a method for video decoding is disclosed. The method may includeextracting a first transform coefficient of a first color component froma bitstream of a coded video; extracting a second transform coefficientof a second color component from the bitstream of the coded video;deriving an offset value based on a magnitude or sign value of the firsttransform coefficient; adding the offset value to a magnitude of thesecond transform coefficient to generate a modified second transformcoefficient for the second color component; and reconstructing the codedvideo based on at least the first transform coefficient of the firstcolor component and the modified second transform coefficient of thesecond color component.

In the implementations above, the first transform coefficient and thesecond transform coefficient are collocated.

In any of the implementations above, the first color component comprisesone chroma component whereas the second color component may includeanother chroma component.

In some of the implementations above, the first color componentcomprises a luma component whereas the second color component mayinclude one chroma component.

In some of the implementations above, the first color componentcomprises one chroma component whereas the second color component mayinclude luma component.

In any of the implementations above, the first transform coefficient isnonzero as quantized from the bitstream; and the second transformcoefficient is zero as quantized from the bitstream.

In any of the implementations above, the first transform coefficientcomprises a sign value and a magnitude value; and deriving the offsetvalue comprises deriving the offset value based on the sign value of thefirst transform coefficient. In some implementations, adding the offsetvalue to the magnitude of the second transform coefficient may includeadding the offset value to the magnitude of the second transformcoefficient after dequantization. In some other implementations, addingthe offset value to the magnitude of the second transform coefficientmay include adding the offset value to the magnitude of the secondtransform coefficient prior to dequantization.

In any of the implementations above, the offset value has an oppositesign to a sign of the first transform coefficient, and adding the offsetvalue to the magnitude of the second transform coefficient may includeadding the offset value to the magnitude of the second transformcoefficient after dequantization.

In any of the implementations above, whether a sign value of the firsttransform coefficient and a sign for the offset value are opposite issignaled in one of a video parameter set (VPS); a sequence parameter set(SPS); a picture parameter set (PPS); an adaptation parameter set (APS);a frame header; a slice header; a coding tree unit header; or a tileheader.

In any of the implementations above, the offset value depends on both asign and a magnitude of the first transform coefficient. In someimplementations, a magnitude of the offset value depends on themagnitude of the first transform coefficient.

In some of the implementations above, a magnitude of the offset value ispredefined via a predetermined correspondence between magnitudes ofoffset values and magnitudes of transform coefficients of the firstcolor component.

In any of the implementations above, the offset value is determineddepending on a frequency position corresponding to the first transformcoefficient.

In any of the implementations above, the offset value is determineddepending on a block size of a transform block that the first transformcoefficient and the second transform coefficient belong to.

In any of the implementations above, the offset value is determineddepending on whether the second color component is a luma component or achroma component.

Aspects of the disclosure also provide a video encoding or decodingdevice or apparatus including a circuitry configured to carry out any ofthe method implementations above.

Aspects of the disclosure also provide non-transitory computer-readablemediums storing instructions which when executed by a computer for videodecoding and/or encoding cause the computer to perform the methods forvideo decoding and/or encoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A shows a schematic illustration of an exemplary subset of intraprediction directional modes.

FIG. 1B shows an illustration of exemplary intra prediction directions.

FIG. 2 shows a schematic illustration of a current block and itssurrounding spatial merge candidates for motion vector prediction in oneexample.

FIG. 3 shows a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an example embodiment.

FIG. 4 shows a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an example embodiment.

FIG. 5 shows a schematic illustration of a simplified block diagram of avideo decoder in accordance with an example embodiment.

FIG. 6 shows a schematic illustration of a simplified block diagram of avideo encoder in accordance with an example embodiment.

FIG. 7 shows a block diagram of a video encoder in accordance withanother example embodiment.

FIG. 8 shows a block diagram of a video decoder in accordance withanother example embodiment.

FIG. 9 shows a scheme of coding block partitioning according to exampleembodiments of the disclosure.

FIG. 10 shows another scheme of coding block partitioning according toexample embodiments of the disclosure.

FIG. 11 shows another scheme of coding block partitioning according toexample embodiments of the disclosure.

FIG. 12 shows another scheme of coding block partitioning according toexample embodiments of the disclosure.

FIG. 13 shows a scheme for partitioning a coding block into multipletransform blocks and coding order of the transform blocks according toexample embodiments of the disclosure.

FIG. 14 shows another scheme for partitioning a coding block intomultiple transform blocks and coding order of the transform blockaccording to example embodiments of the disclosure.

FIG. 15 shows another scheme for partitioning a coding block intomultiple transform blocks according to example embodiments of thedisclosure.

FIG. 16 shows a flow chart of method according to an example embodimentof the disclosure.

FIG. 17 shows a schematic illustration of a computer system inaccordance with example embodiments of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the example of FIG. 3 , the first pair of terminal devices (310) and(320) may perform unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., of a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display the video pictures according tothe recovered video data. Unidirectional data transmission may beimplemented in media serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that perform bidirectionaltransmission of coded video data that may be implemented, for example,during a videoconferencing application. For bidirectional transmissionof data, in an example, each terminal device of the terminal devices(330) and (340) may code video data (e.g., of a stream of video picturesthat are captured by the terminal device) for transmission to the otherterminal device of the terminal devices (330) and (340) via the network(350). Each terminal device of the terminal devices (330) and (340) alsomay receive the coded video data transmitted by the other terminaldevice of the terminal devices (330) and (340), and may decode the codedvideo data to recover the video pictures and may display the videopictures at an accessible display device according to the recoveredvideo data.

In the example of FIG. 3 , the terminal devices (310), (320), (330) and(340) may be implemented as servers, personal computers and smart phonesbut the applicability of the underlying principles of the presentdisclosure may not be so limited. Embodiments of the present disclosuremay be implemented in desktop computers, laptop computers, tabletcomputers, media players, wearable computers, dedicated videoconferencing equipment, and/or the like. The network (350) representsany number or types of networks that convey coded video data among theterminal devices (310), (320), (330) and (340), including for examplewireline (wired) and/or wireless communication networks. Thecommunication network (350)9 may exchange data in circuit-switched,packet-switched, and/or other types of channels. Representative networksinclude telecommunications networks, local area networks, wide areanetworks and/or the Internet. For the purposes of the presentdiscussion, the architecture and topology of the network (350) may beimmaterial to the operation of the present disclosure unless explicitlyexplained herein.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, a placement of a video encoder and a video decoder in avideo streaming environment. The disclosed subject matter may be equallyapplicable to other video applications, including, for example, videoconferencing, digital TV broadcasting, gaming, virtual reality, storageof compressed video on digital media including CD, DVD, memory stick andthe like, and so on.

A video streaming system may include a video capture subsystem (413)that can include a video source (401), e.g., a digital camera, forcreating a stream of video pictures or images (402) that areuncompressed. In an example, the stream of video pictures (402) includessamples that are recorded by a digital camera of the video source 401.The stream of video pictures (402), depicted as a bold line to emphasizea high data volume when compared to encoded video data (404) (or codedvideo bitstreams), can be processed by an electronic device (420) thatincludes a video encoder (403) coupled to the video source (401). Thevideo encoder (403) can include hardware, software, or a combinationthereof to enable or implement aspects of the disclosed subject matteras described in more detail below. The encoded video data (404) (orencoded video bitstream (404)), depicted as a thin line to emphasize alower data volume when compared to the stream of uncompressed videopictures (402), can be stored on a streaming server (405) for future useor directly to downstream video devices (not shown). One or morestreaming client subsystems, such as client subsystems (406) and (408)in FIG. 4 can access the streaming server (405) to retrieve copies (407)and (409) of the encoded video data (404). A client subsystem (406) caninclude a video decoder (410), for example, in an electronic device(430). The video decoder (410) decodes the incoming copy (407) of theencoded video data and creates an outgoing stream of video pictures(411) that are uncompressed and that can be rendered on a display (412)(e.g., a display screen) or other rendering devices (not depicted). Thevideo decoder 410 may be configured to perform some or all of thevarious functions described in this disclosure. In some streamingsystems, the encoded video data (404), (407), and (409) (e.g., videobitstreams) can be encoded according to certain video coding/compressionstandards. Examples of those standards include ITU-T RecommendationH.265. In an example, a video coding standard under development isinformally known as Versatile Video Coding (VVC). The disclosed subjectmatter may be used in the context of VVC, and other video codingstandards.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anyembodiment of the present disclosure below. The video decoder (510) canbe included in an electronic device (530). The electronic device (530)can include a receiver (531) (e.g., receiving circuitry). The videodecoder (510) can be used in place of the video decoder (410) in theexample of FIG. 4 .

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510). In the same or another embodiment,one coded video sequence may be decoded at a time, where the decoding ofeach coded video sequence is independent from other coded videosequences. Each video sequence may be associated with multiple videoframes or images. The coded video sequence may be received from achannel (501), which may be a hardware/software link to a storage devicewhich stores the encoded video data or a streaming source whichtransmits the encoded video data. The receiver (531) may receive theencoded video data with other data such as coded audio data and/orancillary data streams, that may be forwarded to their respectiveprocessing circuitry (not depicted). The receiver (531) may separate thecoded video sequence from the other data. To combat network jitter, abuffer memory (515) may be disposed in between the receiver (531) and anentropy decoder/parser (520) (“parser (520)” henceforth). In certainapplications, the buffer memory (515) may be implemented as part of thevideo decoder (510). In other applications, it can be outside of andseparate from the video decoder (510) (not depicted). In still otherapplications, there can be a buffer memory (not depicted) outside of thevideo decoder (510) for the purpose of, for example, combating networkjitter, and there may be another additional buffer memory (515) insidethe video decoder (510), for example to handle playback timing. When thereceiver (531) is receiving data from a store/forward device ofsufficient bandwidth and controllability, or from an isosynchronousnetwork, the buffer memory (515) may not be needed, or can be small. Foruse on best-effort packet networks such as the Internet, the buffermemory (515) of sufficient size may be required, and its size can becomparatively large. Such buffer memory may be implemented with anadaptive size, and may at least partially be implemented in an operatingsystem or similar elements (not depicted) outside of the video decoder(510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such asdisplay (512) (e.g., a display screen) that may or may not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as is shown in FIG. 5 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received by theparser (520). The entropy coding of the coded video sequence can be inaccordance with a video coding technology or standard, and can followvarious principles, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (520) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameter corresponding to thesubgroups. The subgroups can include Groups of Pictures (GOPs),pictures, tiles, slices, macroblocks, Coding Units (CUs), blocks,Transform Units (TUs), Prediction Units (PUs) and so forth. The parser(520) may also extract from the coded video sequence information such astransform coefficients (e.g., Fourier transform coefficients), quantizerparameter values, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple differentprocessing or functional units depending on the type of the coded videopicture or parts thereof (such as: inter and intra picture, inter andintra block), and other factors. The units that are involved and howthey are involved may be controlled by the subgroup control informationthat was parsed from the coded video sequence by the parser (520). Theflow of such subgroup control information between the parser (520) andthe multiple processing or functional units below is not depicted forsimplicity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these functional units interact closelywith each other and can, at least partly, be integrated with oneanother. However, for the purpose of describing the various functions ofthe disclosed subject matter with clarity, the conceptual subdivisioninto the functional units is adopted in the disclosure below.

A first unit may include the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) may receive a quantized transformcoefficient as well as control information, including informationindicating which type of inverse transform to use, block size,quantization factor/parameters, quantization scaling matrices, and thelie as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values that canbe input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block, i.e., a block that does not usepredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) may generate a block of the same size and shape ofthe block under reconstruction using surrounding block information thatis already reconstructed and stored in the current picture buffer (558).The current picture buffer (558) buffers, for example, partlyreconstructed current picture and/or fully reconstructed currentpicture. The aggregator (555), in some implementations, may add, on aper sample basis, the prediction information the intra prediction unit(552) has generated to the output sample information as provided by thescaler/inverse transform unit (551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forinter-picture prediction. After motion compensating the fetched samplesin accordance with the symbols (521) pertaining to the block, thesesamples can be added by the aggregator (555) to the output of thescaler/inverse transform unit (551) (output of unit 551 may be referredto as the residual samples or residual signal) so as to generate outputsample information. The addresses within the reference picture memory(557) from where the motion compensation prediction unit (553) fetchesprediction samples can be controlled by motion vectors, available to themotion compensation prediction unit (553) in the form of symbols (521)that can have, for example X, Y components (shift), and referencepicture components (time). Motion compensation may also includeinterpolation of sample values as fetched from the reference picturememory (557) when sub-sample exact motion vectors are in use, and mayalso be associated with motion vector prediction mechanisms, and soforth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values. Several type of loop filters may beincluded as part of the loop filter unit 556 in various orders, as willbe described in further detail below.

The output of the loop filter unit (556) can be a sample stream that canbe output to the rendering device (512) as well as stored in thereference picture memory (557) for use in future inter-pictureprediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future inter-picture prediction. For example,once a coded picture corresponding to a current picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, the parser (520)), the current picture buffer(558) can become a part of the reference picture memory (557), and afresh current picture buffer can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology adopted in a standard, suchas ITU-T Rec. H.265. The coded video sequence may conform to a syntaxspecified by the video compression technology or standard being used, inthe sense that the coded video sequence adheres to both the syntax ofthe video compression technology or standard and the profiles asdocumented in the video compression technology or standard.Specifically, a profile can select certain tools from all the toolsavailable in the video compression technology or standard as the onlytools available for use under that profile. To be standard-compliant,the complexity of the coded video sequence may be within bounds asdefined by the level of the video compression technology or standard. Insome cases, levels restrict the maximum picture size, maximum framerate, maximum reconstruction sample rate (measured in, for examplemegasamples per second), maximum reference picture size, and so on.Limits set by levels can, in some cases, be further restricted throughHypothetical Reference Decoder (HRD) specifications and metadata for HRDbuffer management signaled in the coded video sequence.

In some example embodiments, the receiver (531) may receive additional(redundant) data with the encoded video. The additional data may beincluded as part of the coded video sequence(s). The additional data maybe used by the video decoder (510) to properly decode the data and/or tomore accurately reconstruct the original video data. Additional data canbe in the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anexample embodiment of the present disclosure. The video encoder (603)may be included in an electronic device (620). The electronic device(620) may further include a transmitter (640) (e.g., transmittingcircuitry). The video encoder (603) can be used in place of the videoencoder (403) in the example of FIG. 4 .

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the example ofFIG. 6 ) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) may beimplemented as a portion of the electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 YCrCb, RGB, XYZ . . .), and any suitable sampling structure (for example YCrCb 4:2:0, YCrCb4:4:4). In a media serving system, the video source (601) may be astorage device capable of storing previously prepared video. In avideoconferencing system, the video source (601) may be a camera thatcaptures local image information as a video sequence. Video data may beprovided as a plurality of individual pictures or images that impartmotion when viewed in sequence. The pictures themselves may be organizedas a spatial array of pixels, wherein each pixel can comprise one ormore samples depending on the sampling structure, color space, and thelike being in use. A person having ordinary skill in the art can readilyunderstand the relationship between pixels and samples. The descriptionbelow focuses on samples.

According to some example embodiments, the video encoder (603) may codeand compress the pictures of the source video sequence into a codedvideo sequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speedconstitutes one function of a controller (650). In some embodiments, thecontroller (650) may be functionally coupled to and control otherfunctional units as described below. The coupling is not depicted forsimplicity. Parameters set by the controller (650) can include ratecontrol related parameters (picture skip, quantizer, lambda value ofrate-distortion optimization techniques, . . . ), picture size, group ofpictures (GOP) layout, maximum motion vector search range, and the like.The controller (650) can be configured to have other suitable functionsthat pertain to the video encoder (603) optimized for a certain systemdesign.

In some example embodiments, the video encoder (603) may be configuredto operate in a coding loop. As an oversimplified description, in anexample, the coding loop can include a source coder (630) (e.g.,responsible for creating symbols, such as a symbol stream, based on aninput picture to be coded, and a reference picture(s)), and a (local)decoder (633) embedded in the video encoder (603). The decoder (633)reconstructs the symbols to create the sample data in a similar manneras a (remote) decoder would create even though the embedded decoder 633process coded video steam by the source coder 630 without entropy coding(as any compression between symbols and coded video bitstream in entropycoding may be lossless in the video compression technologies consideredin the disclosed subject matter). The reconstructed sample stream(sample data) is input to the reference picture memory (634). As thedecoding of a symbol stream leads to bit-exact results independent ofdecoder location (local or remote), the content in the reference picturememory (634) is also bit exact between the local encoder and remoteencoder. In other words, the prediction part of an encoder “sees” asreference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used to improve coding quality.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5 . Brieflyreferring also to FIG. 5 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633) inthe encoder.

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that may only be presentin a decoder also may necessarily need to be present, in substantiallyidentical functional form, in a corresponding encoder. For this reason,the disclosed subject matter may at times focus on decoder operation,which allies to the decoding portion of the encoder. The description ofencoder technologies can thus be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas or aspects a more detail description of the encoder is providedbelow.

During operation in some example implementations, the source coder (630)may perform motion compensated predictive coding, which codes an inputpicture predictively with reference to one or more previously codedpicture from the video sequence that were designated as “referencepictures.” In this manner, the coding engine (632) codes differences (orresidue) in the color channels between pixel blocks of an input pictureand pixel blocks of reference picture(s) that may be selected asprediction reference(s) to the input picture. The term “residue” and itsadjective form “residual” may be used interchangeably.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end (remote) video decoder (absent transmissionerrors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compression of the symbols accordingto technologies such as Huffman coding, variable length coding,arithmetic coding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person having ordinary skill in the art is aware of thosevariants of I pictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample coding blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16samples each) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures. The sourcepictures or the intermediate processed pictures may be subdivided intoother types of blocks for other purposes. The division of coding blocksand the other types of blocks may or may not follow the same manner, asdescribed in further detail below.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data may accordingly conform to a syntax specified bythe video coding technology or standard being used.

In some example embodiments, the transmitter (640) may transmitadditional data with the encoded video. The source coder (630) mayinclude such data as part of the coded video sequence. The additionaldata may comprise temporal/spatial/SNR enhancement layers, other formsof redundant data such as redundant pictures and slices, SEI messages,VUI parameter set fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) utilizes spatial correlation in a givenpicture, and inter-picture prediction utilizes temporal or othercorrelation between the pictures. For example, a specific picture underencoding/decoding, which is referred to as a current picture, may bepartitioned into blocks. A block in the current picture, when similar toa reference block in a previously coded and still buffered referencepicture in the video, may be coded by a vector that is referred to as amotion vector. The motion vector points to the reference block in thereference picture, and can have a third dimension identifying thereference picture, in case multiple reference pictures are in use.

In some example embodiments, a bi-prediction technique can be used forinter-picture prediction. According to such bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that both proceed the current picture in the video indecoding order (but may be in the past or future, respectively, indisplay order) are used. A block in the current picture can be coded bya first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bejointly predicted by a combination of the first reference block and thesecond reference block.

Further, a merge mode technique may be used in the inter-pictureprediction to improve coding efficiency.

According to some example embodiments of the disclosure, predictions,such as inter-picture predictions and intra-picture predictions areperformed in the unit of blocks. For example, a picture in a sequence ofvideo pictures is partitioned into coding tree units (CTU) forcompression, the CTUs in a picture may have the same size, such as 64×64pixels, 32×32 pixels, or 16×16 pixels. In general, a CTU may includethree parallel coding tree blocks (CTBs): one luma CTB and two chromaCTBs. Each CTU can be recursively quadtree split into one or multiplecoding units (CUs). For example, a CTU of 64×64 pixels can be split intoone CU of 64×64 pixels, or 4 CUs of 32×32 pixels. Each of the one ormore of the 32×32 block may be further split into 4 CUs of 16×16 pixels.In some example embodiments, each CU may be analyzed during encoding todetermine a prediction type for the CU among various prediction typessuch as an inter prediction type or an intra prediction type. The CU maybe split into one or more prediction units (PUs) depending on thetemporal and/or spatial predictability. Generally, each PU includes aluma prediction block (PB), and two chroma PBs. In an embodiment, aprediction operation in coding (encoding/decoding) is performed in theunit of a prediction block. The split of a CU into PU (or PBs ofdifferent color channels) may be performed in various spatial pattern. Aluma or chroma PB, for example, may include a matrix of values (e.g.,luma values) for samples, such as 8×8 pixels, 16×16 pixels, 8×16 pixels,16×8 samples, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherexample embodiment of the disclosure. The video encoder (703) isconfigured to receive a processing block (e.g., a prediction block) ofsample values within a current video picture in a sequence of videopictures, and encode the processing block into a coded picture that ispart of a coded video sequence. The example video encoder (703) may beused in place of the video encoder (403) in the FIG. 4 example.

For example, the video encoder (703) receives a matrix of sample valuesfor a processing block, such as a prediction block of 8×8 samples, andthe like. The video encoder (703) then determines whether the processingblock is best coded using intra mode, inter mode, or bi-prediction modeusing, for example, rate-distortion optimization (RDO). When theprocessing block is determined to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis determined to be coded in inter mode or bi-prediction mode, the videoencoder (703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Insome example embodiments, a merge mode may be used as a submode of theinter picture prediction where the motion vector is derived from one ormore motion vector predictors without the benefit of a coded motionvector component outside the predictors. In some other exampleembodiments, a motion vector component applicable to the subject blockmay be present. Accordingly, the video encoder (703) may includecomponents not explicitly shown in FIG. 7 , such as a mode decisionmodule, to determine the perdition mode of the processing blocks.

In the example of FIG. 7 , the video encoder (703) includes an interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in the examplearrangement in FIG. 7 .

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures in display order), generate inter predictioninformation (e.g., description of redundant information according tointer encoding technique, motion vectors, merge mode information), andcalculate inter prediction results (e.g., predicted block) based on theinter prediction information using any suitable technique. In someexamples, the reference pictures are decoded reference pictures that aredecoded based on the encoded video information using the decoding unit633 embedded in the example encoder 620 of FIG. 6 (shown as residualdecoder 728 of FIG. 7 , as described in further detail below).

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to blocksalready coded in the same picture, and generate quantized coefficientsafter transform, and in some cases also to generate intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). The intra encoder (722) maycalculates intra prediction results (e.g., predicted block) based on theintra prediction information and reference blocks in the same picture.

The general controller (721) may be configured to determine generalcontrol data and control other components of the video encoder (703)based on the general control data. In an example, the general controller(721) determines the prediction mode of the block, and provides acontrol signal to the switch (726) based on the prediction mode. Forexample, when the prediction mode is the intra mode, the generalcontroller (721) controls the switch (726) to select the intra moderesult for use by the residue calculator (723), and controls the entropyencoder (725) to select the intra prediction information and include theintra prediction information in the bitstream; and when the predicationmode for the block is the inter mode, the general controller (721)controls the switch (726) to select the inter prediction result for useby the residue calculator (723), and controls the entropy encoder (725)to select the inter prediction information and include the interprediction information in the bitstream.

The residue calculator (723) may be configured to calculate a difference(residue data) between the received block and prediction results for theblock selected from the intra encoder (722) or the inter encoder (730).The residue encoder (724) may be configured to encode the residue datato generate transform coefficients. For example, the residue encoder(724) may be configured to convert the residue data from a spatialdomain to a frequency domain to generate the transform coefficients. Thetransform coefficients are then subject to quantization processing toobtain quantized transform coefficients. In various example embodiments,the video encoder (703) also includes a residual decoder (728). Theresidual decoder (728) is configured to perform inverse-transform, andgenerate the decoded residue data. The decoded residue data can besuitably used by the intra encoder (722) and the inter encoder (730).For example, the inter encoder (730) can generate decoded blocks basedon the decoded residue data and inter prediction information, and theintra encoder (722) can generate decoded blocks based on the decodedresidue data and the intra prediction information. The decoded blocksare suitably processed to generate decoded pictures and the decodedpictures can be buffered in a memory circuit (not shown) and used asreference pictures.

The entropy encoder (725) may be configured to format the bitstream toinclude the encoded block and perform entropy coding. The entropyencoder (725) is configured to include in the bitstream variousinformation. For example, the entropy encoder (725) may be configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. When coding a block in the merge submode of either inter modeor bi-prediction mode, there may be no residue information.

FIG. 8 shows a diagram of an example video decoder (810) according toanother embodiment of the disclosure. The video decoder (810) isconfigured to receive coded pictures that are part of a coded videosequence, and decode the coded pictures to generate reconstructedpictures. In an example, the video decoder (810) may be used in place ofthe video decoder (410) in the example of FIG. 4 .

In the example of FIG. 8 , the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residual decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in the example arrangement of FIG. 8 .

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (e.g., intra mode, intermode, bi-predicted mode, merge submode or another submode), predictioninformation (e.g., intra prediction information or inter predictioninformation) that can identify certain sample or metadata used forprediction by the intra decoder (872) or the inter decoder (880),residual information in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isthe inter or bi-predicted mode, the inter prediction information isprovided to the inter decoder (880); and when the prediction type is theintra prediction type, the intra prediction information is provided tothe intra decoder (872). The residual information can be subject toinverse quantization and is provided to the residual decoder (873).

The inter decoder (880) may be configured to receive the interprediction information, and generate inter prediction results based onthe inter prediction information.

The intra decoder (872) may be configured to receive the intraprediction information, and generate prediction results based on theintra prediction information.

The residual decoder (873) may be configured to perform inversequantization to extract de-quantized transform coefficients, and processthe de-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residual decoder (873) mayalso utilize certain control information (to include the QuantizerParameter (QP)) which may be provided by the entropy decoder (871) (datapath not depicted as this may be low data volume control informationonly).

The reconstruction module (874) may be configured to combine, in thespatial domain, the residual as output by the residual decoder (873) andthe prediction results (as output by the inter or intra predictionmodules as the case may be) to form a reconstructed block forming partof the reconstructed picture as part of the reconstructed video. It isnoted that other suitable operations, such as a deblocking operation andthe like, may also be performed to improve the visual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In some example embodiments, the video encoders(403), (603), and (703), and the video decoders (410), (510), and (810)can be implemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Turing to coding block partitioning, and in some exampleimplementations, a predetermined pattern may be applied. As shown inFIG. 9 , an example 4-way partition tree starting from a firstpredefined level (e.g., 64×64 block level) down to a second predefinedlevel (e.g., 4×4 level) may be employed. For example, a base block maybe subject to four partitioning options indicated by 902, 904, 906, and908, with the partitions designated as R as being allowed for recursivepartitions in that the same partition tree as indicated in FIG. 9 may berepeated at a lower scale until the lowest level (e.g., 4×4 level). Insome implementations, additional restrictions may be applied to thepartitioning scheme of FIG. 9 . In the implementation of FIG. 9 ,rectangular partitions (e.g., 1:2/2:1 rectangular partitions) may beallowed but they may not be allowed to be recursive, whereas a squarepartitioning is allowed to be recursive. The partitioning following FIG.9 with recursion, if needed, generates a final set of coding blocks.Such scheme may apply to one or more of the color channels.

FIG. 10 shows another example predefined partitioning pattern allowingrecursive partitioning to form a partitioning tree. As shown in FIG. 10, an example 10-way partitioning structure or pattern may be predefined.The root block may start at a predefined level (e.g. from 128×128 level,or 64×64 level). The example partitioning structure of FIG. 10 includesvarious 2:1/1:2 and 4:1/1:4 rectangular partitions. The partition typeswith 3 sub-partitions indicated 1002, 1004, 1006, and 1008 in the secondrow of FIG. 10 may be referred to “T-type” partitions. The “T-Type”partitions 1002, 1004, 1006, and 1008 may be referred to as Left T-Type,Top T-Type, Right T-Type and Bottom T-Type. In some implementations,none of the rectangular partitions of FIG. 10 is allowed to be furthersubdivided. A coding tree depth may be further defined to indicate thesplitting depth from the root node or root block. For example, thecoding tree depth for the root node or root black, e.g. for a 128×128block, may be set to 0, and after the root block is further split oncefollowing FIG. 10 , the coding tree depth is increased by 1. In someimplementations, only the all-square partitions in 1010 may be allowedfor recursive partitioning into the next level of the partitioning treefollowing pattern of FIG. 10 . In other words, recursive partitioningmay not be allowed for the square partitions with patterns 1002, 1004,1006, and 1006. The partitioning following FIG. 10 with recursion, ifneeded, generates a final set of coding blocks. Such scheme may apply toone or more of the color channels.

After dividing or partitioning a base block following any of thepartitioning procedures or other procedures above, again, a final set ofpartitions or coding block may be obtained. Each of these partitions maybe at one of various partitioning levels. Each of the partitions may bereferred to as a coding block (CB). For the various example partitioningimplementations above, each resulting CB may be of any of the allowedsizes and partitioning levels. They are referred to as coding blockbecause they may form units for which some basic coding/decodingdecisions may be made and coding/decoding parameters may be optimized,determined, and signaled in an encoded video bitstream. The highestlevel in the final partitions represents the depth of the coding blockpartitioning tree. Coding block may be a luma coding block or a chromacoding block.

In some other example implementations, a quadtree structure may be usedfor splitting base luma and chroma blocks recursively into coding units.Such splitting structure may be referred to as a coding tree unit (CTU),which is split into coding units (CUs) by using the quadtree structureto adapt the partitioning to various local characteristics of the baseCTU. In such implementations, implicit quadtree split may be performedat picture boundary so that a block will keep quad-tree splitting untilthe size fits the picture boundary. The term CU is used to collectivelyrefer to units of luma and chroma coding blocks (CBs).

In some implementations, a CB may be further partitioned. For example, ACB may be further partitioned into multiple prediction blocks (PBs) forpurposes of intra or inter-frame prediction during coding and decodingprocesses. In other words, a CB may be further divided into differentsub partitions, where individual prediction decision/configuration maybe made. In parallel, a CB may be further partitioned into a pluralityof transform blocks (TBs) for purposes of delineating levels at whichtransform or inverse transform of video data is performed. Thepartitioning scheme of a CB into PBs and TBs may or may not be the same.For example, each partitioning scheme may be performed using its ownprocedure based on, for example, the various characteristics of thevideo data. The PB and TB partitioning schemes may be independent insome example implementations. The PB and TB partitioning schemes andboundaries may be correlated in some other example implementations. Isome implementations, for example, TBs may be partitioned after PBpartitions, and in particular, each PB, after being determined followingpartitioning of a coding block, may then be further partitioned into oneor more TBs. For example, in some implementations, a PB may be splitinto one, two, four, or other number of TBs.

In some implementations, for partitioning of a base block into codingblocks and further into prediction blocks and/or transform blocks, theluma channel and the chroma channels may be treated differently. Forexample, in some implementations, partitioning of a coding block intoprediction blocks and/or transform blocks may be allowed for the lumachannel whereas such partitioning of a coding block into predictionblocks and/or transform blocks may not be allowed for the chromachannel(s). In such implementations, transform and/or prediction of lumablocks thus may be performed only at the coding block level. For anotherexample, minimum transform block size for luma channel and chromachannel(s) may be different, e.g., coding blocks for luma channel may beallowed to be partitioned into smaller transform and/or predictionblocks than the chroma channels. For yet another example, the maximumdepth of partitioning of a coding block into transform blocks and/orprediction blocks may be different between the luma channel and thechroma channels, e.g., coding blocks for luma channel may be allowed tobe partitioned into deeper transform and/or prediction blocks than thechroma channel(s). For a specific example, luma coding blocks may bepartitioned into transform blocks of multiple sizes that can berepresented by a recursive partition going down by up to 2 levels, andtransform block shapes such as square, 2:1/1:2, and 4:1/1:4 andtransform block size from 4×4 to 64×64 may be allowed. For chromablocks, however, only the largest possible transform blocks specifiedfor the luma blocks may be allowed.

In some example implementations for partitioning of a coding block intoPBs, the depth, the shape, and/or other characteristics of the PBpartitioning may depend on whether the PB is intra or inter coded.

The partitioning of a coding block (or a prediction block) intotransform blocks may be implemented in various example schemes,including but not limited to quadtree splitting and predefined patternsplitting, recursively or non-recursively, and with additionalconsideration for transform blocks at the boundary of the coding blockor prediction block. In general, the resulting transform blocks may beat different split levels, may not be of the same size, and may not needto be square in shape (e.g., they can be rectangular with some allowedsizes and aspect ratios).

In some implementations, coding partition tree schemes or structures maybe used. Coding partition tree schemes used for the luma and chromachannels may not need to be the same. In other words, luma and chromachannels may have separate coding tree structures. Further, whether theluma and chroma channels use the same or different coding partition treestructures and the actual coding partition tree structures to be usedmay depend on whether the slice being coded is a P, B, or I slice. Forexample, For an I slice, the chroma channels and luma channel may haveseparate coding partition tree structures or coding partition treestructure modes, whereas for a P or B slice, the luma and chromachannels may share a same coding partition tree scheme. When separatecoding partition tree structures or modes are applied, luma channel maybe partitioned into CBs by one coding partition tree structure, and thechroma channel may be partitioned into chroma CBs by another codingpartition tree structure.

A specific example implementation of coding block and transform blockpartitioning is described below. In such an example implementation, abase coding block may be split into coding blocks using recursivequadtree splitting described above. At each level, whether furtherquadtree splitting of a particular partition should continue may bedetermined by local video data characteristics. The resulting CBs may beat various quadtree splitting levels, of various sizes. The decision onwhether to code a picture area using inter-picture (temporal) orintra-picture (spatial) prediction may be made at the CB level (or CUlevel, for all three-color channels). Each CB may be further split intoone, two, four, or other number of PBs according to PB splitting type.Inside one PB, the same prediction process may be applied and therelevant information is transmitted to the decoder on a PB basis. Afterobtaining the residual block by applying the prediction process based onthe PB splitting type, a CB can be partitioned into TBs according toanother quadtree structure similar to the coding tree for the CB. Inthis particular implementation, a CB or a TB may but does not have to belimited to square shape. Further in this particular example, a PB may besquare or rectangular shape for an inter-prediction and may only besquare for intra-prediction. A coding block may be further split into,e.g., four square-shaped TBs. Each TB may be further split recursively(using quadtree split) into smaller TBs, referred to as ResidualQuad-Tree (RQT).

Another specific example for partitioning of a base coding block intoCBs and other PBs and or TBs are described below. For example, ratherthan using a multiple partition unit types such as those shown in FIG.10 , a quadtree with nested multi-type tree using binary and ternarysplits segmentation structure may be used. The separation of the CB, PBand TB concepts (i.e., the partitioning of CB into PBs and/or TBs, andthe partitioning of PBs into TBs) may be abandoned except when neededfor CBs that have a size too large for the maximum transform length,where such CBs may need further splitting. This example portioningscheme may be designed to support more flexibility for CB partitionshapes so that the prediction and transform can both be performed on theCB level without further partitioning. In such a coding tree structure,a CB may have either a square or rectangular shape. Specifically, acoding tree block (CTB) may be first partitioned by a quadtreestructure. Then the quadtree leaf nodes may be further partitioned by amulti-type tree structure. An example of the multi-type tree structureis shown in FIG. 11 . Specifically, the example multi-type treestructure of FIG. 11 includes four splitting types, referred to asvertical binary splitting (SPLIT_BT_VER) (1102), horizontal binarysplitting (SPLIT_BT_HOR) (1104), vertical ternary splitting(SPLIT_TT_VER) (1106), and horizontal ternary splitting (SPLIT_TT_HOR)(1108). The CBs then corresponds to leaves of the multi-type tree. Inthis example implementation, unless the CB is too large for the maximumtransform length, this segmentation is used for both prediction andtransform processing without any further partitioning. This means that,in most cases, the CB, PB and TB have the same block size in thequadtree with nested multi-type tree coding block structure. Theexception occurs when maximum supported transform length is smaller thanthe width or height of the colour component of the CB.

One example for the quadtree with nested multi-type tree coding blockstructure of block partition for one CTB is shown in FIG. 12 . In moredetail, FIG. 12 shows that the CTB 1200 is quadtree split into foursquare partitions 1202, 1204, 1206, and 1208. Decision to further usethe multi-type tree structure of FIG. 11 for splitting is made for eachof the quadtree-split partitions. In the example of FIG. 12 , partition1204 is not further split. Partitions 1202 and 1208 each adopt anotherquadtree split. For partition 1202, the second level quadtree-splittop-left, top-right, bottom-left, and bottom-right partitions adoptsthird level splitting of quadtree, 1104 of FIG. 11 , non-splitting, and1108 of FIG. 11 , respective. Partition 1208 adopts another quadtreesplit, and the second level quadtree-split top-left, top-right,bottom-left, and bottom-right partitions adopts third level splitting of1106 of FIG. 11 , non-splitting, non-splitting, and 1104 of FIG. 11 ,respectively. Two of the subpartitions of the third-level top-leftpartition of 1208 are further split according to 1104 and 1108.Partition 1206 adopts a second level split pattern following 1102 ofFIG. 11 into two partitions which are further split in a third-levelaccording to 1108 and 1102 of the FIG. 11 . A fourth level splitting isfurther applied to one of them according to 1104 of FIG. 11 .

For the specific example above, the maximum luma transform size may be64×64 and the maximum supported chroma transform size could be differentfrom the luma at, e.g., 32×32. When the width or height of the lumacoding block or chroma coding block is larger than the maximum transformwidth or height, the luma coding block or chroma coding block may beautomatically split in the horizontal and/or vertical direction to meetthe transform size restriction in that direction.

In the specific example for partitioning of a base coding block into CBsabove, the coding tree scheme may support the ability for the luma andchroma to have a separate block tree structure. For example, for P and Bslices, the luma and chroma CTBs in one CTU may share the same codingtree structure. For I slices, for example, the luma and chroma may haveseparate coding block tree structures. When separate block tree modesare applied, luma CTB may be partitioned into luma CBs by one codingtree structure, and the chroma CTBs are partitioned into chroma CBs byanother coding tree structure. This means that a CU in an I slice mayconsist of a coding block of the luma component or coding blocks of twochroma components, and a CU in a P or B slice always consists of codingblocks of all three colour components unless the video is monochrome.

Example implementations for partitioning a coding block or predictionblock into transform blocks, and a coding order of the transform blocksare described in further detail below. In some example implementations,a transform partitioning may support transform blocks of multipleshapes, e.g., 1:1 (square), 1:2/2:1, and 1:4/4:1, with transform blocksizes ranging from, e.g., 4×4 to 64×64. In some implementations, if thecoding block is smaller than or equal to 64×64, the transform blockpartitioning may only apply to luma component, such that for chromablocks, the transform block size is identical to the coding block size.Otherwise, if the coding block width or height is greater than 64, thenboth the luma and chroma coding blocks may be implicitly split intomultiples of min (W, 64)×min (H, 64) and min (W, 32) x min (H, 32)transform blocks, respectively.

In some example implementations, for both intra and inter coded blocks,a coding block may be further partitioned into multiple transform blockswith a partitioning depth up to a predefined number of levels (e.g., 2levels). The transform block partitioning depth and sizes may berelated. An example mapping from the transform size of the current depthto the transform size of the next depth is shown in the following inTable 1.

TABLE 1 Transform partition size setting Transform Size of TransformSize Current Depth of Next Depth TX_4X4 TX_4X4 TX_8X8 TX_4X4 TX_16X16TX_8X8 TX_32X32 TX_16X16 TX_64X64 TX_32X32 TX_4X8 TX_4X4 TX_8X4 TX_4X4TX_8X16 TX_8X8 TX_16X8 TX_8X8 TX_16X32 TX_16X16 TX_32X16 TX_16X16TX_32X64 TX_32X32 TX_64X32 TX_32X32 TX_4X16 TX_4X8 TX_16X4 TX_8X4TX_8X32 TX_8X16 TX_32X8 TX_16X8 TX_16X64 TX_16X32 TX_64X16 TX_32X16

Based on the example mapping of Table 1, for 1:1 square block, the nextlevel transform split may create four 1:1 square sub-transform blocks.Transform partition may stop, for example, at 4×4. As such, a transformsize for current depth of 4×4 corresponds to the same size of 4×4 forthe next depth. In the example of Table 1, for 1:2/2:1 non-square block,the next level transform split will create two 1:1 square sub-transformblocks, whereas for 1:4/4:1 non-square block, the next level transformsplit will create two 1:2/2:1 sub transform blocks.

In some example implementations, for luma component of an intra codedblock, additional restriction may be applied. For example, for eachlevel of transform partitioning, all the sub-transform blocks may berestricted to having equal size. For example, for a 32×16 coding block,level 1 transform split creates two 16×16 sub-transform blocks, level 2transform split creates eight 8×8 sub-transform blocks. In other words,the second level splitting must be applied to all first level sub blocksto keep the transform units at equal sizes. An example of the transformblock partitioning for intra coded square block following Table 1 isshown in FIG. 13 , together with coding order illustrated by the arrows.Specifically, 1302 shows the square coding block. A first-level splitinto 4 equal sized transform blocks according to Table 1 is shown in1304 with coding order indicated by the arrows. A second-level split ofall of the first-level equal sized blocks into 16 equal sized transformblocks according to Table 1 is shown in 1306 with coding order indicatedby the arrows.

In some example implementations, for luma component of inter codedblock, the above restriction for intra coding may not be applied. Forexample, after the first level of transform splitting, any one ofsub-transform block may be further split independently with one morelevel. The resulting transform blocks thus may or may not be of the samesize. An example split of an inter coded block into transform locks withtheir coding order is show in FIG. 14 . In the Example of FIG. 14 , theinter coded block 1402 is split into transform blocks at two levelsaccording to Table 1. At the first level, the inter coded block is splitinto four transform blocks of equal size. Then only one of the fourtransform blocks (not all of them) is further split into foursub-transform blocks, resulting in a total of 7 transform blocks havingtwo different sizes, as shown by 1404. The example coding order of these7 transform blocks is shown by the arrows in 1404 of FIG. 14 .

In some example implementations, for chroma component(s), someadditional restriction for transform blocks may apply. For example, forchroma component(s) the transform block size can be as large as thecoding block size, but not smaller than a predefined size, e.g., 8×8.

In some other example implementations, for the coding block with eitherwidth (W) or height (H) being greater than 64, both the luma and chromacoding blocks may be implicitly split into multiples of min (W, 64)×min(H, 64) and min (W, 32)×min (H, 32) transform units, respectively.

FIG. 15 further shows another alternative example scheme forpartitioning a coding block or prediction block into transform blocks.As shown in FIG. 15 , instead of using recursive transform partitioning,a predefined set of partitioning types may be applied to a coding blockaccording a transform type of the coding block. In the particularexample shown in FIG. 15 , one of the 6 example partitioning types maybe applied to split a coding block into various number of transformblocks. Such scheme may be applied to either a coding block or aprediction block.

In more detail, the partitioning scheme of FIG. 15 provides up to 6partition types for any given transform type as shown in FIG. 15 . Inthis scheme, every coding block or prediction block may be assigned atransform type based on, for example, a rate-distortion cost. In anexample, the partition type assigned to the coding block or predictionblock may be determined based on the transform partition type of thecoding block or prediction block. A particular partition type maycorrespond to a transform block split size and pattern (or partitiontype), as shown by the 4 partition types illustrated in FIG. 15 . Acorrespondence relationship between various transform types and thevarious partition types may be predefined. An example correspondence isshown below with the capitalized labels indicating the transform typesthat may be assigned to the coding block or prediction block based onrate distortion cost:

-   -   PARTITION_NONE: Assigns a transform size that is equal to the        block size.    -   PARTITION_SPLIT: Assigns a transform size that is ½ the width of        the block size and ½ the height of the block size.    -   PARTITION_HORZ: Assigns a transform size with the same width as        the block size and ½ the height of the block size.    -   PARTITION_VERT: Assigns a transform size with ½ the width of the        block size and the same height as the block size.    -   PARTITION_HORZ4: Assigns a transform size with the same width as        the block size and ¼ the height of the block size.    -   PARTITION_VERT4: Assigns a transform size with ¼ the width of        the block size and the same height as the block size.

In the example above, the partition types as shown in FIG. 15 allcontain uniform transform sizes for the partitioned transform blocks.This is a mere example rather than a limitation. In some otherimplementations, mixed transform blocks sizes may be used for thepartitioned transform blocks in a particular partition type (orpattern).

Turning to some example implementations of a particular type ofsignaling for coding blocks/units, for each intra and inter coding unit,a flag, namely skip_txfm flag, may be signaled in the coded bitstream,as shown in the example syntax of Table 2 and represented by theread_skip( ) function for the retrieval of these flag from thebitstream. This flag may indicate whether the transform coefficients areall zero in the current coding unit. In some example implementations, ifthis flag is signaled with, for example, a value 1, then anothertransform coefficient related syntaxes, e.g., EOB (End of Block) neednot be signaled for any of the color coding blocks in the coding unit,and can be derived as a value or data structure predefined for andassociated with zero transform coefficients block. For inter codingblock, as shown by the example of Table 2, this flag may be signaledafter a skip mode flag, which indicate that the coding unit may beskipped for various reasons. When skip_mode is true, the coding unitshould be skipped and there is no need to signal any skip_txfm flag andthe skip_texfm flag is inferred as 1. Otherwise, if skip_mode is false,then more information about the coding unit would be included in thebitstream and skip_txfm flag would be additionally signaled to indicatewhether the coding unit is all zero or not.

TABLE 2 Skip Mode and Skip Syntax Type Intra frame mode info syntax intra_frame_mode_info( ) {    skip = 0    if ( SegIdPreSkip )      intra_segment_id( )    skip_mode = 0    read_skip( )    if (!SegIdPreSkip )       intra_segment_id( )    read_cdef( )   read_delta_qindex( )    read_delta_lf( )    ReadDeltas = 0   RefFrame[ 0 ] = INTRA_FRAME    RefFrame[ 1 ] = NONE    if (allow_intrabc ) {       use_intrabc S( )    } else {       use_intrabc =0    }     Intra frame mode info syntax  inter_frame_mode_info( ) {  use_intrabc = 0   LeftRefFrame[ 0 ] = AvailL ? RefFrames[ MiRow ][MiCol−1 ][ 0 ] : INTRA_FRAME   AboveRefFrame[ 0 ] = AvailU ? RefFrames[MiRow−1 ][ MiCol ][ 0 ] : INTRA_FRAME   LeftRefFrame[ 1 ] = AvailL ?RefFrames[ MiRow ][ MiCol−1 ][ 1 ] : NONE   AboveRefFrame[ 1 ] = AvailU? RefFrames[ MiRow−1 ][ MiCol ][ 1 ] : NONE   LeftIntra = LeftRefFrame[0 ] <= INTRA_FRAME   AboveIntra = AboveRefFrame[ 0 ] <= INTRA_FRAME  LeftSingle = LeftRefFrame[ 1 ] <= INTRA_FRAME   AboveSingle =AboveRefFrame[ 1 ] <= INTRA_FRAME   skip = 0   inter_segment_id( 1 )  read_skip_mode( )   if ( skip_mode )      skip = 1   else     read_skip( )     Skip syntax  read_skip( ) {    if ( SegIdPreSkip&& seg_feature_active( SEG_LVL_SKIP ) ) {       skip = 1    } else {      skip S( )    }  }

In the example implementations below, the term chroma channel maygenerally refers to both Cb and Cr color components (or channels), orboth U and V color components (or channels). The term luma channel mayinclude luma component, or Y component. Luma component or channel may bereferred to as luma color component or channel. Y, U and V are usedbelow to denote the three color components. Further, the term “codedblock” and “coding” block are used interchangeably to mean either ablock to be coded or a block already coded. They may be a block of anyof the three color components. The three corresponding colorcoded/coding blocks may for a coded/coding unit.

Turning to coding and decoding (entropy coding) of transformcoefficients of residuals in each of the color component, for eachtransform block, transform coefficient coding may start with signalingof the skip sign, followed by the transform kernel type and theend-of-block (EOB) position when the skip sign is zero (indicating thereare nonzero coefficients). Then each coefficient value is mapped tomultiple level maps (amplitude map) and the sign.

After the EOB position is coded, the lower-level map and themiddle-level map may be coded in reverse scan order, the formerindicating if the coefficient magnitude is within a low level (e.g.,between 0 and 2) while the latter indicating if the range is within amiddle level (e.g., between 3 and 14). The next step codes, in theforward-scanning order, the sign of the coefficients as well as theresidual values of the coefficient larger than a high level (e.g., 14)by, for example, Exp-Golomb code.

As for the use of context modeling, the lower-level map coding mayincorporate the transform size and directions as well as up to fiveneighboring coefficient information. On the other hand, the middle-levelmap coding may follow a similar approach as with the lower-levelamplitude coding except that the number of neighboring coefficients isdown to a smaller number (e.g., two) The example Exp-Golomb code for theresidual level as well as the sign of AC coefficients are coded withoutany context model while the sign of DC coefficients is coded using itsneighbor transform-block's dc sign.

In some example implementations, the chroma residuals may coded jointly.Such a coding scheme may be based on some statistical correlationbetween the chroma channels. For example, in many cases, the Cr and Cbchroma coefficient may be similar in amplitudes and opposite in sign,and thus on, for example, a transform block level, where transformcoefficients are signaled, may be jointed encoded to improve codingefficiency by only introducing small color distortion. The usage(activation) of a joint chroma coding mode may, for example indicated bya joint chroma coding flag (e.g., TU-level flagtu_joint_cbcr_residual_flag) and the selected joint mode may beimplicitly indicated by the chroma CBFs.

Specifically, the flag tu_joint_cbcr_residual_flag may be present ifeither or both chroma CBFs for a TU (transform block) are equal to 1. Inthe PPS and slice header, chroma quantization parameter (QP) offsetvalues may be signalled for the joint chroma residual coding mode todifferentiate from the chroma QP offset values signalled for regularchroma residual coding mode. These chroma QP offset values may be usedto derive the chroma QP values for those blocks coded using the jointchroma residual coding mode. When a corresponding joint chroma codingmode (modes 2 in Table 3) is active in a TU, this chroma QP offset maybe added to the applied luma-derived chroma QP during quantization anddecoding of that TU. For the other modes (modes 1 and 3 in Table 3), thechroma QPs may be derived in the same way as for conventional Cb or Crblocks. The reconstruction process of the chroma residuals (resCb andresCr) from the transmitted transform blocks is depicted in Table 3.When this mode is activated (mode 2), one single joint chroma residualblock (resJointC[x][y] in Table 3) may be signalled, and residual blockfor Cb (resCb) and residual block for Cr (resCr) may be derivedconsidering information such as tu_cbf cb, tu_cbf cr, and CSign, whichis a sign value specified in, for example, the slice header rather thanat the transform block level. In some implementations, CSign may be −1most of he time.

The three example joint chroma coding modes described above may be onlysupported in intra coded CU. In inter-coded CU, only mode 2 may besupported. Hence, for inter coded CU, the syntax elementtu_joint_cbcr_residual_flag is only present if both chroma CBFs are 1.

TABLE 1 Reconstruction of chroma residuals. The value CSign is a signvalue (+1 or −1), which is specified in the slice header, resJointC[ ][] is the transmitted residual. tu_cbf_cb tu_cbf_cr reconstruction of Cband Cr residuals mode 1 0 resCb[ x ][ y ] = resJointC[ x ][ y ] 1 resCr[x ][ y ] = ( CSign * resJointC[ x ][ y ] ) >> 1 1 1 resCb[ x ][ y ] =resJointC[ x ][ y ] 2 resCr[ x ][ y ] = CSign * resJointC[ x ][ y ] 0 1resCb[ x ][ y ] = ( CSign * 3 resJointC[ x ][ y ] ) >> 1 resCr[ x ][ y ]= resJointC[ x ][ y ]

The joint chroma coding scheme above assume some correlation between thetransform coefficients between collocated Cr and Cb transform blocks.These assumptions are usually statistical and thus may bring distortionin some situations. In particular, when one of the color coefficients inthe transform block is non-zero whereas another color component has zerocoefficient, then some of the assumption made in joint chroma codingscheme would certainly be off and such coding would not save any codedbits either (because one of the chroma coefficients are zero anyway).

In the various example implementations below, a coefficient level (thatis, transform coefficient by transform coefficient) cross-componentcoding scheme is described that takes advantage of some correlationbetween collocated transform coefficients (collocated in frequencydomain) of color components. Such a scheme is particularly useful forthe transform blocks (or units) where coefficients of one colorcomponent are zero whereas corresponding transform coefficients ofanother color components are nonzero. For those pairs of zero andnon-zero color coefficients, either before or after dequantization, thenonzero color coefficient may be used to estimate the original smallvalue of the zero coded coefficient of the other color component (whichmay originally be nonzero albeit small value before quantization in thecoding process), thereby possibly recovering some information lost in,for example, the quantization process during encoding. Some lostinformation during quantization to zero may be recovered because of theinter-color correlation that statistically exists. Such cross-componentcoding would recover lost information to some extent without significantcoding cost (of the zero coefficients).

In particular, across-component coefficient sign coding method may beimplemented, which utilizes the coefficient sign value of a first colorcomponent to code the coefficient sign of a second color component. Inone more specific example, the sign value of a Cb transform coefficientmay be used as the context for coding the sign other Cr transformcoefficient. Such cross-component coding may be implemented on transformcoefficient pairs of the color components, coefficient by coefficient.The principle underlying such implementation and other implementationsdescribed in further detail below are not limited to Cr and Cbcomponents. They are applicable between any two of the three colorcomponents. In that respect, the luma channel is considered one of thecolor components.

The example implementations below may be used separately or combined inany order. The term block size may refer to either the block width orheight, or maximum value of width and height, or minimum of width andheight, or area size (width*height), or aspect ratio (width:height, orheight:width) of the block. The term “level value” or “level” may referto the magnitude of the transform coefficient value.

In some example implementations, the level value and/or sign value ofthe transform coefficient of a first color component may be used toderive an offset value that is added to the transform coefficient levelvalue of a second color component.

In some further implementations, the transform coefficient of the firstcolor component used generate the offset and the second color componentare co-located (same coordinate in frequency domain, e.g., the estimateis not-cross frequency).

While the first color component and the second color component describedabove may not be limited to particular color component, in some exampleimplementations, the first color component maybe Cb (or Cr) whereas thesecond color component is Cr (or Cb).

In some specific example implementations, the first color component maybe luma, the second color component may be one of Cb and Cr.

In some specific example implementations, the first color component maybe one of Cb and Cr, and the second color component may be luma.

In some example implementations, the quantized transform coefficient ofthe first color component may be nonzero, and the quantized transformcoefficient of the said second color component may be zero. As such theoriginal relatively small non-zero information of the original transformcoefficient of the second component may be lost due to quantizationduring encoding process, and the example implementations describedherein help recover some lost information using a corresponding non-zerocolor component that may be statistically correlated with thezero-coefficient color component.

In some example implementations, the sign value of the transformcoefficient of the first color component may be used to derive an offsetvalue that is added to the dequantized transform coefficient level valueof the second color component.

In some example implementations, the sign value of the transformcoefficient of the first color component is used to derive an offsetvalue that is added to the transform coefficient level value of thesecond color component before dequantization.

In some example implementations, if the sign value of the transformcoefficient of the first color component is positive (or negative), anegative (or positive) offset value is added to the dequantizedtransform coefficient level value of the second color component forreconstructing the transform coefficient value of a second colorcomponent. In other words, the sign value of the transform coefficientof the first color component and the sign value of the offset value thatis added to the transform coefficient level value of the second colorcomponent have different sign values. Such implementations may beconsistent with statistical observations that two chroma componenttypically have opposite signs for transform coefficients.

In some example implementations, whether the sign value of the transformcoefficient of the first color component and the sign value of theoffset value that is added to the transform coefficient level value ofthe second color component has opposite sign value is signaled inhigh-level syntax, include but not limited to: SPS, VPS, PPS, APS,picture header, frame header, slice header, tile header, CTU header.This is similar to the signaling scheme described above for the jointchroma coding scheme.

In some example implementations, the offset value that is added to thetransform coefficient level value of the second color component maydepend on both the sign and level of the transform coefficient of thefirst color component.

In some example implementations, the magnitude of the offset value thatis added to the transform coefficient level value of the second colorcomponent may depend on the transform coefficient level of the firstcolor component.

In some example implementations, the magnitude of the offset value thatis added to the transform coefficient level value of the second colorcomponent may be pre-defined for each input value of coefficient levelof the transform coefficient of the first color component.

In some example implementations, the offset value that is added to thetransform coefficient level value of the second color component maydepend on frequency that the transform coefficient is located at. Forexample, the offset value may be smaller for higher frequencycoefficients.

In some example implementations, the offset value that is added to thetransform coefficient level value of the second color component maydepend on the block size of the block that the transform coefficientbelongs to. For example, the offset value may generally be smaller forlarger block size.

In some example implementations, the offset value that is added to thetransform coefficient level value of the second color component maydepend on whether the second component is a luma (Y) or a chroma (Cb orCr) component. For example, the offset value may be smaller if thesecond color component is luma.

FIG. 16 shows a flow chart 1600 of an example method following theprinciples underlying the implementations above for cross componentdecoding. The example method flow starts at 1601. In S1610, a firsttransform coefficient of a first color component is extracted from abitstream of a coded video. In S1620, a second transform coefficient ofa second color component is extracted from the bitstream of the codedvideo. In S1630, an offset value is derived based on a magnitude or signvalue of the first transform coefficient. In S1640, the offset value isadded to a magnitude of the second transform coefficient to generate amodified second transform coefficient for the second color component. InS1650, the coded video is reconstructed based on at least the firsttransform coefficient of the first color component and the modifiedsecond transform coefficient of the second color component. The examplemethod flow ends at S1699.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium. Embodiments in the disclosure may be appliedto a luma block or a chroma block.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1722),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include an interface (1754) to one ormore communication networks (1755). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CAN bus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general-purpose data ports or peripheral buses (1749) (such as,for example USB ports of the computer system (1700)); others arecommonly integrated into the core of the computer system (1700) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (1700) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), graphicsadapters (1750), and so forth. These devices, along with Read-onlymemory (ROM) (1745), Random-access memory (1746), internal mass storagesuch as internal non-user accessible hard drives, SSDs, and the like(1747), may be connected through a system bus (1748). In some computersystems, the system bus (1748) can be accessible in the form of one ormore physical plugs to enable extensions by additional CPUs, GPU, andthe like. The peripheral devices can be attached either directly to thecore's system bus (1748), or through a peripheral bus (1749). In anexample, the screen (1710) can be connected to the graphics adapter(1750). Architectures for a peripheral bus include PCI, USB, and thelike.

CPUs (1741), GPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As a non-limiting example, the computer system having architecture(1700), and specifically the core (1740) can provide functionality as aresult of processor(s) (including CPUs, GPUs, FPGA, accelerators, andthe like) executing software embodied in one or more tangible,computer-readable media. Such computer-readable media can be mediaassociated with user-accessible mass storage as introduced above, aswell as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (2640). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(2640) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (2646) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (2644)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

APPENDIX A: ACRONYMS

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUL Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

HDR: high dynamic rangeSDR: standard dynamic range

JVET: Joint Video Exploration Team

MPM: most probable mode

WAIP: Wide-Angle Intra Prediction CU: Coding Unit PU: Prediction UnitTU: Transform Unit CTU: Coding Tree Unit PDPC: Position DependentPrediction Combination ISP: Intra Sub-Partitions SPS: Sequence ParameterSetting PPS: Picture Parameter Set APS: Adaptation Parameter Set VPS:Video Parameter Set DPS: Decoding Parameter Set ALF: Adaptive LoopFilter SAO: Sample Adaptive Offset CC-ALF: Cross-Component Adaptive LoopFilter CDEF: Constrained Directional Enhancement Filter CCSO:Cross-Component Sample Offset LSO: Local Sample Offset LR: LoopRestoration Filter AV1: AOMedia Video 1 AV2: AOMedia Video 2

What is claimed is:
 1. A method for video decoding, comprising:extracting a first transform coefficient of a first color component froma bitstream of a coded video; extracting a second transform coefficientof a second color component from the bitstream of the coded video;deriving an offset value based on a magnitude or sign value of the firsttransform coefficient; adding the offset value to a magnitude of thesecond transform coefficient to generate a modified second transformcoefficient for the second color component; and reconstructing the codedvideo based on at least the first transform coefficient of the firstcolor component and the modified second transform coefficient of thesecond color component.
 2. The method of claim 1, wherein the firsttransform coefficient and the second transform coefficient arecollocated.
 3. The method of claim 1, wherein the first color componentcomprises one chroma component whereas the second color componentcomprise another chroma component.
 4. The method of claim 1, wherein thefirst color component comprises a luma component whereas the secondcolor component comprises one chroma component.
 5. The method of claim1, wherein the first color component comprises one chroma componentwhereas the second color component comprises luma component.
 6. Themethod of claim 1, wherein: the first transform coefficient is nonzeroas quantized from the bitstream; and the second transform coefficient iszero as quantized from the bitstream.
 7. The method of claim 1, wherein:the first transform coefficient comprises a sign value and a magnitudevalue; and deriving the offset value comprises deriving the offset valuebased on the sign value of the first transform coefficient.
 8. Themethod of claim 7, wherein adding the offset value to the magnitude ofthe second transform coefficient comprises adding the offset value tothe magnitude of the second transform coefficient after dequantization.9. The method of claim 7, wherein adding the offset value to themagnitude of the second transform coefficient comprises adding theoffset value to the magnitude of the second transform coefficient priorto dequantization.
 10. The method of claim 1, wherein: the offset valuehas an opposite sign to a sign of the first transform coefficient; andadding the offset value to the magnitude of the second transformcoefficient comprises adding the offset value to the magnitude of thesecond transform coefficient after dequantization.
 11. The method ofclaim 1, wherein whether a sign value of the first transform coefficientand a sign for the offset value are opposite is signaled in one of: avideo parameter set (VPS); a sequence parameter set (SPS) a pictureparameter set (PPS); an adaptation parameter set (APS); a frame header;a slice header; a coding tree unit header; or a tile header.
 12. Themethod of claim 1, wherein the offset value depends on both a sign and amagnitude of the first transform coefficient.
 13. The method of claim12, wherein a magnitude of the offset value depends on the magnitude ofthe first transform coefficient.
 14. The method of claim 12, wherein amagnitude of the offset value is predefined via a predeterminedcorrespondence between magnitudes of offset values and magnitudes oftransform coefficients of the first color component.
 15. The method ofclaim 1, wherein the offset value is determined depending on a frequencyposition corresponding to the first transform coefficient.
 16. Themethod of claim 1, wherein the offset value is determined depending on ablock size of a transform block that the first transform coefficient andthe second transform coefficient belong to.
 17. The method of claim 1,wherein the offset value is determined depending on whether the secondcolor component is a luma component or a chroma component.
 18. A devicefor video decoding, comprising a circuitry configured to: extract afirst transform coefficient of a first color component from a bitstreamof a coded video; extract a second transform coefficient of a secondcolor component from the bitstream of the coded video; derive an offsetvalue based on a magnitude or sign value of the first transformcoefficient; add the offset value to a magnitude of the second transformcoefficient to generate a modified second transform coefficient for thesecond color component; and reconstruct the coded video based on atleast the first transform coefficient of the first color component andthe modified second transform coefficient of the second color component.19. A device of claim 18, wherein: a quantized coefficient of the firsttransform coefficient is nonzero; and a quantized coefficient of thesecond transform coefficient is zero.
 20. A non-transitory computerreadable medium for storing computer instructions, the computerinstructions, when executed by a processor, causes the processor to:extract a first transform coefficient of a first color component from abitstream of a coded video; extract a second transform coefficient of asecond color component from the bitstream of the coded video; derive anoffset value based on a magnitude or sign value of the first transformcoefficient; add the offset value to a magnitude of the second transformcoefficient to generate a modified second transform coefficient for thesecond color component; and reconstruct the coded video based on atleast the first transform coefficient of the first color component andthe modified second transform coefficient of the second color component.